1. Field of the Invention
This invention relates to a laser beam scanning type ophthalmological instrument for scanning an eye fundus by laser beam coming from a laser beam light source in order to observe the eye fundus and to inspect the view field, etc.
2. Prior Art
A conventional laser beam scanning type ophthalmological instrument is used as an eye fundus camera in which a visible laser beam is emitted by a laser beam light source, an eye fundus is scanned by the visible laser beam, a reflecting light reflected on the eye fundus is received by a light receiving system and the eye fundus is displayed on a display screen according to the light received by the light receiving system so that the eye fundus can be observed, or as a subjective optometer in which projection of a laser beam is stopped for a scanning period of time in a desired position of the eye fundus and a target mark such as a landolt ring is formed in that position.
In the conventional laser beam scanning type ophthalmological instrument, however, since a laser beam for scanning the eye fundus is a visible light, the intensity of the laser beam is adjusted as weak as possible in order not to give an unpleasant feeling such as a dazzling to a person to be tested. Because of the foregoing adjustment, a sufficient reflecting light quantity is unobtainable when the reflectance is low at the eye fundus. This naturally makes it very difficult to observe the eye fundus. In order to avoid this undesirable problem to occur, there has been proposed a device, as disclosed for example in Japanese Pat. Early Laid-open Publication No. Sho 62-117524, in which an invisible light is utilized in order to render a certain quantity of background illumination or auxiliary illumination.
However, since the light quantity of the auxiliary illumination is constant in the above device, sometimes a satisfactory eye fundus image is unobtainable depending on conditions of the quantity of light which is used for stimulating the eye fundus for example, because there occurs a partial saturation and/or a shortage of light quantity.